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Sunday, August 15, 2010

1. Intro
CAPTCHA Completely Automated Public Turing test to tell Computers and Humans Apart
Invented at CMU by Luis von Ahn, Manuel Blum, et. al
A program that is a challenge – response test to separate humans from computer programs

2. Generic CAPTCHAs
distort letters and numbers
Distorted characters are presented to user
User has to recognize the distorted letters
If the guessed letters are correct, the user is inferred to be a human and allowed access
Else, user is a bot and denied access

3. Humans can read the distorted and noisy text
Current OCRs cannot read them

5. Altavista first used a crude CAPTCHA in their sites
Resulted in 95% spam reduction
Yahoo partnered CMU to counter these threats in Messenger chat service.
Luis von Ahn and Manuel Blum of CMU trademarked CAPTCHA in 2000

6. What is a Turing test?
Proposed by Alan Turing
To test a machine’s level of intelligence
Human judge asks questions to two participants, one is a machine, he doesn’t know which is which
If judge can’t tell which is the machine, the machine passes the test
CAPTCHA employs a reverse Turing test,
judge = CAPTCHA program,
participant = user
if user passes CAPTCHA, he is human
if user fails, it is a machine

7. Types of CAPTCHAs
Text based:
Simple, normal language questions:
What is sum of three and thirty-five?
If today is Saturday, what is day after tomorrow?
Which of mango, table, water is a fruit?
Very effective, needs a large question bank
Cognitively challenged users find it hard

8. Gimpy:
Designed by Yahoo and CMU
Picks up 10 random words from dictionary and distorts, fills with noise
User has to recognize at least 3 words
If user is correct, he is admitted

9. EZ-Gimpy:
A modified version of Gimpy
Yahoo used this version in Messenger
Has only 1 random string of characters
Not a dictionary word, so not prone to dictionary attack
Not a good implementation, already broken by OCRs